Pressure applicator for surface generating apparatus

ABSTRACT

Apparatus for controlling the distribution of pressure between a lapping tool and a work surface being treated. The apparatus comprises structure for applying a predetermined force at a desired position on the moving lap in order to obtain desired pressure characteristics between the lap and the work piece. The structure may be employed to compensate for non-uniform surfacing rates that would otherwise be caused due to the transverse movement of the lap over the work piece. Alternatively, it can be employed to intentionally introduce localized pressure differentials at particular areas between the lap and the work piece to assist in obtaining desired surface characteristics.

llnited States Patent [191 Mcflallum [451 Feb. 12, 1974 PRESSURE APPLICATOR FOR SURFACE GENERATING APPARATUS [75] lnventor: James B. McCallum, Plymouth,

Mass.

[73] Assignee: ltek Corporation, Lexington, Mass.

[22] Filed: May 23, 1972 [21] Appl. No: 256,052

[52] US. Cl. 51/56, 51/120 7 [51] llnt. Cl B241) 7/22 58] fieiiiors'earhp; ..5 l/l65,165.9,ll7,121, 5l/129-131,119,284, 120

[56] References (Iited UNITED STATES PATENTS 7/1960 Lipkins 51/117 8/1969 Wright 11/1959 Boettcher 11/ 1959 Boettcher 51/131 2,772,521 12/1956 Voytech 51/131 Primary Examiner-Othell M. Simpson Attorney, Agent, or Firml-lomer 0. Blair; Robert L.

Nathans; Gerald H. Glanzman 57 ABSTRACT Apparatus for controlling the distribution of pressure between a lapping tool and a work surface being treated. The apparatus comprises structure for applying a predetermined force at a desired position on the h moving lap in order to obtain desired pressure characteristics between the lap and the work piece. The structure may be employed to compensate for nonuniform surfacing rates that would otherwise be caused due to the transverse movement of the lap over the work piece. Alternatively, it can be employed to intentionally introduce localized pressure differentials at particular areas between the lap and the work piece to assist in obtaining desired surface characteristics' 13 Claims, 4 Drawing Figures 43 39 r PNEUMATIC TRANSLATIONAL ROTATlONAL PRESSURE DRIVE DRIVE SOURCE MOTOR MOTOR PRESSURE APPLICATOR FOR SURFACE GENERATING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to apparatus for generating optical or other surfaces. More specifically, the present invention relates to apparatus for controlling the application of pressure between a lapping tool and a work surface being treated.

2. Description of the Prior Art It is known that when a relatively flat object is slid across a surface, frictional forces between the object and the surface will tend to cause the moving object to tilt forwardly toward its direction of movement. This same characteristic is experienced when a lapping tool is moved over the surface of a lens blank or other work piece for the purpose of grinding or polishing it. In such a process, the act of simply moving the lap transversely over the surface of the work piece causes the forward end of the moving lap to tilt and, as a result, to press harder against the lens blank surface than its trailing end. Because it is pressing harder, the leading end of the lap will also grind or polish the surface at a faster rate than the trailing end and this can affect the quality as well as the shape of the surface being treated. When substantial accuracy is required, in fact, some form of compensatory action must generally be taken to minimize the effects of the non-uniform grinding or polishing rate so that the required surface characteristics may be obtained.

One practice that is sometimes employed to obtain a more uniform grinding or polishing rate at all points between the lap and the work surface is to preform the shape of the lapping tool face. By accurately designing the face of the tool, the frictional forces can be adjusted to ensure a moreuniform rate of material removal. The problem with this technique, however, is that the lap is more expensive to initially construct and,

I in addition, must be frequently trued.

A second compensating technique often employed is to periodically reverse the positions of the lap and the lens blank during the surfacing process so that each will move over the other for a period of time. This process results in an undesirable waste of operator time and is also more likely to introduce alignment inaccuracies.

A third method that is effective in certain types of lapping operations is to simply design the lapping program to provide the lap with apath of movement that will eventually eliminate all surfacing errors. Such programs, however, usually must be quite complicated and, accordingly, increase the surfacing time required.

Finally, by merely pressing the lap down harder against the work surface, the effect of the transverse forces may be reduced. This technique, however, increases the wear rate of the surfacing tool and is unsuitable in treating thin or delicate optical elements.

In general, although some of the above techniques are useful and may be used to reduce errors in specific surfacing operations, they are not universally effective for the wide variety of lapping operations that are conventionally performed.

SUMMARY OF THE PREFERRED EMBODIMENT OF THE INVENTION In accordance with the preferred embodiment of the present invention, many of the deficiencies of the prior art are obviated by providing a novel pressure controlapparatus that will maintain the pressure distribution between the lapping tool and the work surface substan tially uniform throughout an entire lapping operation, and, in this way, ensure that the grinding or polishing rate will be substantially the same at all areas beneath the lapping tool. In accordance with one preferred embodiment of the invention, the pressure control apparatus comprises a pressure applicator adapted to be pressed against the moving lap (or the work piece when it is the movable member) and apply a force at a position and of a magnitude to counterbalance the uneven pressure distribution introduced by the lateral motion of the lap. In a conventional operation, this will be accomplished by applying the force adjacent the trailing end of the moving lap. In some applications, the pressure applicator may be mounted to move with and follow the tool while in other operations it is sufficient to rigidly mount it to the machine frame.

Several significant advantages are provided by the pressure control apparatus of the present invention. Initially, accurate lapping characteristics are much easier to maintain and control than in the prior art. By being able to maintain the pressure distribution more uniform, it often becomes possible to use larger sized laps which, in turn, reduces the total surfacing time required. In addition, the ability to maintain a uniform pressure may permit less complex surfacing paths to be followed thus simplifying the programming of the lap movement.

The provision of the pressure applicator also provides the operator with an additional tool which can be used in planning the surfacing operation. For example, in certain grinding operations, it mightbe desirable to intentionally introduce localized forces between the grinding tool and the work piece in order to obtain specific surface characteristics. For example, the intentional addition of a force at a precise position would often permit an aspheric surface to be ground more readily than byprior art methods. The present invention is particularly suitable for such specialized uses.

In general, the present invention provides the designer with an added parameter which may be employed along with other controls to perform desired surfacing operations in a more accurate and economical manner. The apparatus itself is quite simple in design and may readily be incorporated into conventional lapping equipment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates a conventional lapping operation as an aid in understanding the present invention.

FIG. 2 illustrates a lapping operation employing the pressure control apparatus according to a preferred embodiment of the present invention.

FIG. 3 illustrates a second embodiment of the present invention.

FIG. 4 illustrates a further use of the pressure control apparatus of the present invention for specialized lapping operations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT To assist in understanding the present invention, FIG. 1 is provided to schematically illustrate a conventional lapping operation. Reference number identifies a work piece which may be a lens blank or other optical or non-optical element having a surface 11 upon which desired characteristics are to be provided as by polishing or grinding although, for ease in explanation, the discussion hereinafter will primarily describe grinding operations. This work piece will generally be mounted to a suitable support 12 which may be either stationary or capable of movement as recognized in the art.

A lapping tool, schematically represented by the number 13 is provided to perform the desired operations on surface 11 by moving in some prescribed path over the surface. This lapping tool may be provided with a suitable abrasive face to directly perform the polishing or grinding or, alternatively, it may be designed to distribute abrasive slurry over the surface 11 in a manner well known in the art. Usually, the lap will also be rapidly rotated about its axis.

In any event, in a conventional lapping operation, the lap is first pressed down against surface 11 with a force F represented by arrow 16 in FIG. 1. This force represents the resultant force due to the weight of the lap pressing down against the work piece and is applied substantially axially of the lap as shown, although, if desired, force F, might also include some additional positive force to increase the pressure of the lap against surface 11. In order to move the lap 13 transversely over surface 11 it is also necessary to apply a second force F; substantially transverse to surface 11. This force is indicated by and will cause the lap to move in the direction of arrow 17. As can be recognized from FIG. 1, forces F and F will both be applied to surface 11 by the lap 13 and the vector sum of those forces will be a resultant force F applied in the direction of arrow 18. As can also be readily understood, the effect of the resultant force F will tend to tilt the lap forwardly toward its direction of movement and this will cause the leading end 19 of the lap to press down harder against surface 11 than the trailing end 21 of the lap. Because it is pressing harder, leading end 19 will also grind surface 11 at a faster rate than trailing end 21, and unless compensated for in one way or another, undesirable surface characteristics may be produced. It is toward this problem that the present invention is primarily directed and FIG. 2 illustrates a preferred embodiment that will effectively solve it.

The embodiment illustrated in FIG. 2 is particularly designed for lapping operations in which the lapping tool is moved along a relatively small path during which it regularly overrides the edge of the element being ground. Pressure differentials become particularly significant at these times and, in fact, the trailing end of the lap may actually leave the work piece surface unless properly compensated for.

As illustrated in FIG. 2, a lapping tool 31 of conventional type is provided for the purpose of grinding a desired contour onto surface 32 of a work piece 33 such as a lens blank. The lens blank is supported by appropriate structure 34 in a manner known in the art. Tool 31 is provided with an appropriate lapping face 36 which may be formed of a suitable abrasive for directly grinding surface 32 or it may be of a material for distributing abrasive slurry over the surface. Tool 31 further includes a spindle 37 through which it is coupled, by appropriate mechanical connections 38 and 42, to appropriate drive motors 39 and 43, respectively. Drive motor 39 is employed to rotate the lap during the grinding operation while motor 42 is adapted to move the lap over the surface 32 of the optical element in a desired path utilizing conventional structure not shown.

In the embodiment of FIG. 2, it will be assumed that the lap is adapted to be driven back and forth across surface 32 as indicated by the arrows 44 and 46. As explained previously, movement of the lap in the direction of arrow 44 will normally tend to cause the forward end 47 of the lap to tilt downwardly and apply a greater pressure against the surface 32 of the lens blank than the trailing end 48 and hence, grind the blank at a faster rate than the trailing end. In order to compensate for this non-uniform pressure distribution, the pressure control device according to the present invention is provided. This pressure control device is identified generally by reference number 51 and includes a shaft 52 which is adapted to be pressed against lap 31. A ball bearing 53 is universally mounted to the lower end of the shaft 52 to provide a substantially frictionless point contact between the shaft and the lap to minimize interference with the rotation and other movements of the lap.

Lap 31 may be of conventional type and generally does not require modification for effective operation of the present invention, however, it may be preferable to provide it with a rubber plate 61 and a metal plate 62 as shown to act as a pressure distributer and prevent the weight of shaft 52 from producing localized deformations on the lap.

Coupled to the shaft 52 and maintaining it in proper position relative to the lap is an arm 54. This arm is, in turn, coupled through a hinge connection 56 to a sleeve 57 which is slidably mounted to a post 58. Sleeve 57 may be slid up and down on post 58 to adjust the position of assembly 51 and a lock nut 59 is provided to lock the sleeve in the desired position. The post 58 is preferably coupled to the machine frame illustrated schematically at 63, and, if desired, may be capable of being adjusted relative to the lap by movement in the direction of arrows 64.

The action of pressure control apparatus 51 according to the present invention illustrated in FIG. 2 can be best understood by briefly describing its effect during a grinding operation. Initially, the lap is positioned adjacent the optical element 33 to be surfaced. The lap may press down on the surface due to its own weight or additional force may be applied axially of the spindle 37 as is known to those skilled in the art.

Let it be assumed that the lap is to move over the sur face 32 in the direction of arrow 44. The pressure control member 52 is initially positioned relative to the lap such that it will press down on it substantially near the center of the lap as shown in solid lines when the entire lapping surface is in contact with surface 32. Shaft 52 is pressed down against lap 31 with the desired force which may be accomplished simply by adding dead weights to the shaft. It is preferred, however, to adjust the force from a source of pneumatic pressure 66 through a valve 67 to enable a finer degree of accuracy to be obtained. The amount of force to be applied by shaft 52 is a function of several variables including the speed of movement of the lap, its size, and the characteristics of the abrasive material used, and can be readily determined by simple experimentation as understood by those skilled in the art.

In operation, when drive motor 43 is actuated to move the lap in the direction of arrow 44, the previ-. ously described tilting tendency will be encountered. In this embodiment, however, wherein the lap surface is large compared to the surface being ground, the tilting forces will not be very great when the entire surface of the lap is resting against the work piece and, therefore, it will be sufficient that the pressure control member be located substantially at the center or prehaps slightly toward the trailing end of center during these periods. As the lap moves in the direction of arrow 44, however, it will soon override the edge of the work piece as indicated in dotted lines in FIG. 2 and, when this happens, the pressure differentials between the leading and trailing edges of the lap will become quite significant and, in fact, the trailing end of the lap may actually leave the surface of the work piece completely. But, as can be seen in FIG. 2, the pressure control member 51 will prevent this from occurring. As the lap moves farther and farther in the direction of arrow 44, the weighted shaft 52 will move closer and closer to the trailing end of the lap and apply pressure thereaginst. Thus, although the tilting tendency of the lap will increase as it overrides the work piece by increasing amounts, the pressure applicator will apply its force increasingly closer to the trailing end of the lap, and, in this way, maintain the pressure of the lap against the work piece substantially uniform at all points to ensure a substantially uniform grinding rate to all areas of the work piece surface.

Obviously, pressure control member 51 illustrated in FIG. 2 is effective only to compensate for non-uniform pressures created by motion of the lap in the direction of arrow 44. If it is necessary or desirable to compensate for lap motion in the direction of arrow 46 a second pressure control apparatus identical to apparatus 51 may be provided to cooperate with the end 47 of the lap. In this reqard, appropriate control circuitry coupled to drive motor 43 could be provided to move pressure control member 51 or the other member into contact with or away from the lap whenever the lap changes its direction of movement. In this way uniform pressures may be obtained for lap movement in both directions and a more uniform surface may be obtained on the work piece 31.

In some grinding operations, particularly in the grinding of very large, high precision optical elements a lap is moved over the surface of the optical element in a precise, carefully designed path in order to gradually produce the desired surface contour. Surfacing operations of this type are illustrated in U. S. Pat. Nos. 3,587,195 and 3,589,078. As can be seen in these patents, the lap is moved in an irregular path and changes direction often during the process. In operations such as these, the embodiment of FIG. 2 would not be effective and the embodiment illustrated in FIG. 3 would be preferred. FIG. 3 is similar in many respects to that shown in F IG. 2 except that the pressure control apparatus is mounted to follow along with the lap as it moves over the surface of the work piece. As shown in FIG. 3, shaft 71 is coupled directly to the spindle 72 of a lap 73 by means of a sleeve 74 having a suitable lock nut 76 for rigidly securing the shaft to the lap in the desired position. The amount of force applied by the shaft to the lap may be adjusted as desired in the manner previously described.

With this form of construction, as the lap moves in an irregular path over the surface 77 of the work piece 78, with end 79 of the lap always being the leading edge, the pressure control member will constantly apply a counteracting force to the trailing end 81 of the lap to counteract the tilting tendency of the leading end and in this way maintain a substantially uniform pressure between the lap and the work piece at all times.

To this point, the pressure control apparatus of the present invention has been described primarily as a tool to compensate for any non-uniform pressure distribution introduced by the lap movement over a work piece. It should also be recognized, however, that the apparatus has wider application than this. It can also be effectively used as an additional tool to obtain any desired pressure distribution between a lap and a work piece surface and in this way simplify many grinding operations. One possible application is illustrated in FIG. 4 which shows a work piece 91 having a substantially spherical work surface 92 which is to be given an aspherical curvature as shown in dotted lines. The lap 93 also has a spherical lapping surface with the same curvature as surface 92. With the present invention, the surface can readily be given its desired shape by positioning the weighted shaft such that it will continuously apply a predetermined force between the lap and the work piece at a position substantially axial of the work piece as illustrated by the arrow F. By maintaining this localized force throughout the grinding operation, the lap will continuously apply a greater force against central areas of the work piece than peripheral areas, and, accordingly, grind the center at a faster rate than peripheral areas. In this way, the desired aspheric shape may be readily generated.

This is one example only of how the apparatus may be used in specialized surfacing operations. Many other applications will readily be recognized by those skilled in the art. In general, the pressure control apparatus of the present invention may be employed as an additional parameter by the optical designer in many surfacing operations to simplify or reduce the grinding time that would normally be required.

In conclusion, the present invention provides apparatus that will enable precise control over the distribution of pressure between a lapping tool and a work piece in many different types of grinding or polishing operations. The apparatus may be used to correct otherwise introduced pressure differentials or to produce localized areas of increased pressure. The device may also readily be associated with either the lap or the work piece as desired. The structure is quite simple in design and operation, higher flexible in its use and can readily be incorporated into conventional lapping equipment.

It should be cearly understood that the above embodiments are meant to be exemplary only and that many modifications will readily present themselves to those skilled in the art. Accordingly, it is intended that the invention is to be limited only as required by the scope of the following claims.

I claim:

1. Surface generating apparatus comprising:

a. a work piece member having a work surface to be operated upon;

b. a lapping tool member having a lapping face for operating upon said work piece surface;

c. first force applying means for pressing the work surface of said work piece member and the lapping face of said lapping tool member against one another with a first force;

. second force applying means for applying a second force between said work piece and lapping tool members for transversely moving one of said members relative to the other of said members; and

e. pressure control means for pressing the work surface of said work piece member and the lapping face of said lapping tool member against one another with a third force spaced from said first force for controlling the pressure distribution between said lapping face and said work piece surface, said pressure control means including means for pressing the movable one of said members against the other of said members with said third force and further including means for adjusting the position at which said third force applying means applies said third force to the movable one of said members.

2. Surface generating apparatus as recited in claim 1 wherein said position adjusting means includes means for varying the position at which said third force applying means applies said third force in accordance with the relative positions of said members.

3. Surface generating apparatus as recited in claim 1 wherein said pressure control means further includes means for adjusting the magnitude of said third force applied by said third force applying means.

4. Surface generating apparatus as recited in claim 3 wherein said force magnitude adjusting means comprises fluid pressure control means.

5. Surface generating apparatus as recited in claim 1 wherein said third force applying means includes substantially frictionless bearing means for contacting the movable one of said members and for applying said third force to said movable member without interfering with its movement.

6. Surface generating apparatus as recited in claim 5 wherein said bearing means comprises a ball bearing.

7. Surface generating apparatus as recited in claim 1 and further including means for coupling said third force applying means to the movable one of said members for movement therewith.

8. Surface generating apparatus as recited in claim 1 wherein said third force applying means applies said third force against the movable one of said members substantially parallel to said first force and spaced therefrom.

9. Surface generating apparatus as recited in claim 8 wherein said third force applying means applies said third force against a trailing portion of said movable member opposite its direction of movement for compensating for any non-uniform pressure distribution between the lapping tool face and said workpiece surface created by the vector sum of said first and second forces.

10. Surface generating apparatus as set forth in claim 9 wherein said movable one of said members comprises the lapping tool member.

11. Surface generating apparatus comprising in combination:

a. a work piece having a work surface to be operated upon;

b. a lapping tool having a lapping face for operating upon said work surface;

c. means for pressing the lapping face of said lapping tool against said work surface with a first force;

d. drive means for applying a second force to said lapping tool for moving it transversely over the surface of said work piece; and

e. pressure control means for applying a third force against said lapping tool at a position spaced from said first force for additionally pressing said lapping face against said work piece surface for controlling the distribution of pressure between said lapping face and said work piece surface.

12. Surface generating apparatus as recited in claim 11 wherein said pressure control means includes means for applying said third force against the end of said lapping tool opposite its direction of movement over said work piece surface.

13. Surface generating apparatus as recited in claim 12 wherein said pressure control means applies said third force at a position and of a magnitude to compensate for any non-uniformity in the distribution of pressure between the work piece surface and the lapping face created by the vector sum of said first and second forces. 

1. Surface generating apparatus comprising: a. a work piece member having a work surface to be operated upon; b. a lapping tool member having a lapping face for operating upon said work piece surface; c. first force applying means for pressing the work surface of said work piece member and the lapping face of said lapping tool member against one another with a first force; d. second force applying means for applying a second force between said work piece and lapping tool members for transversely moving one of said members relative to the other of said members; and e. pressure control means for pressing the work surface of said work piece member and the lapping face of said lapping tool member against one another with a third force spaced from said first force for controlling the pressure distribution between said lapping face and said work piece surface, said pressure control means including means for pressing the movable one of said members against the other of said members with said third force and further including means for adjusting the position at which said third force applying means applies said third force to the movable one of said members.
 2. Surface generating apparatus as recited in claim 1 wherein said position adjusting means includes means for varying the position at which said third force applying means applies said third force in accordance with the relative positions of said members.
 3. Surface generating apparatus as recited in claim 1 wherein said pressure control means further includes means for adjusting the magnitude of said third force applied by said third force applying means.
 4. Surface generating apparatus as recited in claim 3 wherein said force magnitude adjusting means comprises fluid pressure control means.
 5. Surface generating apparatus as recited in claim 1 wherein said third force applying means includes substantially frictionless bearing means for contacting the movable one of said members and for applying said third force to said movable member without interfering with its movement.
 6. Surface generating apparatus as recited in claim 5 wherein said bearing means comprises a ball bearing.
 7. Surface generating apparatus as recited in claim 1 and further including means for coupling said third force applying means to the movable one of said members for movement therewith.
 8. Surface generating apparatus as recited in claim 1 wherein said third forcE applying means applies said third force against the movable one of said members substantially parallel to said first force and spaced therefrom.
 9. Surface generating apparatus as recited in claim 8 wherein said third force applying means applies said third force against a trailing portion of said movable member opposite its direction of movement for compensating for any non-uniform pressure distribution between the lapping tool face and said workpiece surface created by the vector sum of said first and second forces.
 10. Surface generating apparatus as set forth in claim 9 wherein said movable one of said members comprises the lapping tool member.
 11. Surface generating apparatus comprising in combination: a. a work piece having a work surface to be operated upon; b. a lapping tool having a lapping face for operating upon said work surface; c. means for pressing the lapping face of said lapping tool against said work surface with a first force; d. drive means for applying a second force to said lapping tool for moving it transversely over the surface of said work piece; and e. pressure control means for applying a third force against said lapping tool at a position spaced from said first force for additionally pressing said lapping face against said work piece surface for controlling the distribution of pressure between said lapping face and said work piece surface.
 12. Surface generating apparatus as recited in claim 11 wherein said pressure control means includes means for applying said third force against the end of said lapping tool opposite its direction of movement over said work piece surface.
 13. Surface generating apparatus as recited in claim 12 wherein said pressure control means applies said third force at a position and of a magnitude to compensate for any non-uniformity in the distribution of pressure between the work piece surface and the lapping face created by the vector sum of said first and second forces. 